The invention relates to an ultrasonic cleaning method for cleaning films, scales and sludge deposits from surfaces of assemblies after their exposure to high temperature water or steam and, more particularly, to a method for cleaning the surfaces of industrial process vessels such as shell and tube heat exchangers and the like.
Metallic surfaces exposed to water or aqueous solutions over long periods of time in closed heat transfer systems tend to develop films or scales and/or become covered by sludge regardless of the system purity levels. Thus, for example, in commercial electric power generating plants, after several months of on-line operation at high temperatures of 200xc2x0 C. or more, large vessels such as shell and tube heat exchangers commonly known as steam generators tend to develop adherent films, scales and/or sludge deposits on the surfaces of tubes, tubesheets, tube support plates and other internal structural parts even though the purity of the water may be controlled to the parts per million level or lower. These films, scales and sludge will after a period of time have an adverse affect on the operational performance of the steam generators. Also, in pressurized water nuclear power plants for generating commercial electric power, radioactive films tend to develop on the internal surfaces of channel heads of steam generators or other primary system components even though the purity of the pressurized water is controlled at the parts per billion level. Undesirably, such radioactive films may raise background radiation levels in a plant.
Various off-line cleaning methods have been developed to remove the films, scales and sludges which have built up on the internal surfaces of heat exchangers which generate steam. Commercially successful methods include: pressure pulsing with shock waves; water slapping; chemical cleaning; sludge lancing; use of scale conditioning agents; and/or flushing with very large volumes of water. However, these off-line methods, including setup and various other auxiliary operations, invariably require long periods of time on critical path schedules to remove adherent films, scales and sludges which buildup on tube surfaces near to tubesheets and tube support sheets. Also, adherent scales which develop in annular crevices between tube external surfaces and tube support plates and siliceous sludge piles which buildup on tubesheets have proven to be especially difficult to remove. Thus, residual amounts of adherent scale and sludge which can not be removed are permitted to remain on the internal surfaces of the steam generators at the end of a commercial cleaning step, which reduces the effectiveness of the cleaning operation. The residual scale and sludge problem is compounded by an industry trend toward ever faster refueling outages, which limits the window of time available for cleaning operations.
Accordingly, the power generation industry and its suppliers have long searched for practical methods which will more effectively attack adherent films, scales and/or sludges built up on the internals of steam generators. Thus, about twenty years ago (when U.S. Pat. No. 4,320,528 was filed) or more, it was proposed to employ ultrasonics alone or in connection with known chemical cleaning compositions in commercial nuclear reactor systems in order to remove the buildup of corrosion, oxidation and sedimentation on tubes, tubesheets and tube support plates of steam generators. However, ultrasonic techniques have not heretofore proven to be commercially satisfactory for cleaning the many interior rows of closely spaced, small diameter tubes extending from tubesheets and support plates on the secondary (or shell) side of steam generators. According to U.S. Pat. No. 4,645,542, the placement of transducers in the steam generators in accordance with the practice of U.S. Pat. No. 4,320,528 requires considerable time, effort and expense. Also, in some cases it is necessary to cut away a portion of the steam generators, which many owners are reluctant to do (according to the patent).
It is an object of the present invention to provide an effective method for ultrasonically cleaning adherent films, scales and sludges from assembly surfaces previously exposed to water or steam at temperatures of 200xc2x0 C. or more. It is a further object to provide a commercially effective cleaning method.
With these objects in view, the present invention resides in ultrasonic cleaning method, including the steps of: introducing an ultrasonic transducer into a vessel containing an assembly having a surface which was previously exposed to water or steam at temperatures of 200xc2x0 C. or more and has at least part of its surface covered by a film, scale or sludge; submerging the ultrasonic transducer and at least a portion of the surface in a liquid; and generating ultrasonic energy at a power level of at least about 20 watts per gallon at the external surface of the transducer in contact with the liquid and at a frequency of from about 10 KHz to about 200 KHz for introducing the ultrasonic energy into the liquid. In a preferred practice, the energy is generated at a power level of at least about 10 watts/inch2. Most preferably, an array of transducers is employed to output at least 20 to 60 watts per gallon (5-16 watts/liter) of liquid in the vessel. Advantageously, the films, scales and sludges are disrupted without generating large vibrations in the assembly or otherwise structurally damaging the assembly in the course of the cleaning operation. Also, although some large particles may break off the film, scale and sludge, the intense energy tends to produce small particulates which have long settling times in the turbulent liquid flowing in the vessel so that the particulates may be transported away from the surface of the assembly to, e.g., external filter systems where the particulates can be separated from the liquid, and the filtered liquid then recirculated to the surface.
In a preferred practice of the present invention, an ultrasonic transducer or an array of transducers may be introduced into a vessel and suspended in the vessel (and then detached after the cleaning operation). Thus, for example, the transducer or a chain of transducers may be suspended from a tube support plate within the secondary side of a steam generator and hang into a tube bundle. The chain may hang down to the region above the upper surface of the next lower tube support plate or may hang through flow slots in the tube support plates to the region above a more remote tube support plate or even down to the region above the tubesheet. On the primary side of a steam generator, a transducer or transducer chain may be suspended from the tubesheet and hang into a channel head. In a variation of this practice, a transducer or transducer chain may remain in the steam generator during on-line power operations instead of being introduced and removed from the steam generator at the beginning and end of a cleaning operation. Advantageously, large arrays of transducers may be assembled (and later disassembled) in situ in the vessel from smaller subassemblies of transducers which will fit through relatively small nozzles in the vessel. In a similar practice, the transducer or transducer chain may be introduced into a vessel and supported by a removable support assembly instead of by the vessel.
In another preferred practice of the present invention, an ultrasonic transducer or an array of transducers is introduced into a vessel; and the ultrasonic transducer or transducer array and at least a portion of assembly is submerged in a liquid. Ultrasonic energy is then introduced into the liquid by the transducer or transducer array, the transducer or transducer array is moved, and ultrasonic energy is again introduced into the liquid by the transducer or transducer array. Advantageously, the high energy nodes of the ultrasonic transducer or the high energy nodes of the ultrasonic transducer array can be moved through the liquid and clean remote surfaces such as the interior rows of tubes and surrounding area in the assembly. In this practice, the transducer or transducer array may move through the liquid at a speed of about 0.1 inch/minute (2.5 mm/min) or faster while simultaneously introducing energy into the liquid. In other operations, the transducer or transducer array may move through the liquid only between waves.
In preferred commercial practices of the present invention, the cleaning liquid is water or a dilute aqueous solution containing cleaning agents or scale conditioning agents.